In actual use, the wind blowing against the blade, in particular one surface of the blade, rotates the hub with respect to the second axis, thus transferring kinetic energy from the wind to the hub. Wind action on the blade causes electric charges to accumulate along the outer surface of the blade, particularly the tip. Large numbers of electric charges are also accumulated on the blade as a result of lightning striking the wind power system, and in particular the blade.
In both cases, the electric charges must be grounded, which generates electric current flow through the wind power system.
A wind power system has areas, such as the rolling bearings, that are highly sensitive to electric current, which must therefore be prevented from flowing through these areas.
A wind power system normally has at least one rolling bearing between the blade and the hub; a rolling bearing between the hub and the nacelle; and a rolling bearing between the nacelle and the tower. These rolling bearings deteriorate when subjected to electric current generated by electric charges on the blades, and particularly strong electric current produced by lightning striking the blade.
A device must therefore be provided to guide electric current away from the sensitive parts of the wind power system, as described in documents DE 4436290, DE 19826086, DE 19501267, DE 4445899, WO 00/14405, WO 96/07825, and WO 2004/044419.
Each of the above documents describes a safety device designed to guide electric current along a path that does not go through any of the sensitive parts of the wind power system. None of the devices described, however, allows for dimensional variations in the wind power system, caused by tolerances, slack, and deformation, to which both the blade and nacelle are subject. Moreover, the blade is designed to rotate about the third axis to adjust the attack angle of the blade; in which case, the distance between the blade and the hub, and therefore electric connection of the wind power system safety device, also varies, so that the wind power system is no longer adequately protected.